1. Field of the Invention
The present invention relates to a contamination-inspecting apparatus for inspecting a semiconductor wafer, and specifically it relates to a technique for detecting a signal.
2. Description of the Related Art
A contamination-inspecting apparatus for inspecting a surface of a semiconductor wafer for a contaminant has: an irradiation optical system for irradiating a wafer surface with laser light; a detection optical system for detecting light scattered from the wafer surface; a detection unit for converting the scattered light detected with the detection optical system into electric signals to make a correction; and a display unit for making a judgment on a contaminant to display the result. In the detection unit, a detection circuit is incorporated; usually a signal is output through the detection circuit to make a judgment on a contaminant. Now, with reference to FIG. 5, a detection circuit of a semiconductor wafer contamination-inspecting apparatus, which was considered as a related art by the inventor, will be described in outline. The conventional contamination-inspecting apparatus irradiates a wafer with laser light while rotating the wafer, and detects light scattered and reflected by a contaminant on a wafer and the like by use of PMT (Photo Multiplier Tube) 1. Specifically, the PMT 1 is a device which multiplies incident light by the effect of multiplication of photoelectrons, and then outputs an electric current depending on the intensity of incident light. PMT 1 outputs a contamination-detection signal current based on light scattered and reflected by a contaminant and the like. In the detection part 10, an I-V conversion module 2 including a resistor 4 and an operational amplifier 5 converts the contamination-detection signal current into a detection voltage signal. Further, in the detection circuit 3, a logarithmic amplifier 6 logarithmically amplifies the detection voltage signal making an input voltage 8 to the detection circuit 3 so that a minute voltage can be detected even with the minimum resolution of an analog-to-digital conversion circuit 7, and then the analog-to-digital conversion circuit 7 converts the amplified voltage to a digital value. Subsequently, the resultant digital signal having the digital value is output to a data processing circuit (not shown) as an output code 9, and then a judgment about a contaminant or the like is made.
In general, the intensity of light scattered from a contaminant with a minute particle size is said to be proportional to about the sixth power of the diameter of the contaminant. Therefore, in the case where a wafer targeted for inspection is irradiated with light, and scattered light produced by the irradiation is used to detect a contaminant, a detection circuit for detecting a contaminant is required to have a wider dynamic range, which depends on the range of the amplitudes of detected signals. This is because the smaller the contaminant on a wafer is, the minuter the detected signal is. Further, when the number of rotations of a wafer is increased in order to shorten the time for inspecting the wafer, scanning of a surface of the wafer is speeded up, which results in an increase in the rate of change of the detected signal per hour. Therefore, it is required to speed up the operation of the detection part.
Now, as for an analog-to-digital conversion circuit, there is a basic trade-off relation between its processing speed and dynamic range in general. Therefore, the dynamic range of the analog-to-digital conversion circuit tends to decrease with an increase in the speed, and its minimum resolution has a tendency to increase.
Hence, a method to widen the dynamic range by means of arranging amplifiers in parallel, which amplify a detection current with different amplification factors, and using, of outputs of the amplifiers, an output of an appropriate range as the technique disclosed by JP-A-8-145899 has been proposed in the art.